256 



Physiology. 



a near object the lens becomes thicker, and when we look 

 at a distant object the lens becomes less thick. This ad- 

 justment is called accommodation. 



CILIARY MUSCLE 



FAR NEAR CILIARY PROCESS 



Fig. 98. Changes of the Lens in Accommodation. 



Defects of Eyesight In old age the lens usually be- 

 comes less elastic, and cannot adjust for near sight. Since 

 it is unable to grow more convex, artificial lenses (eye- 

 glasses) may be used to enable one to see near objects 

 clearly. Most elderly people see fairly well at a distance, 

 but use glasses for reading or any close work. In " near- 



(2) Far-sighted Eye. (I) Normal Eye. 



Fig. 99. Defects in Eyesight. 



(3) Near-sighted Eye. 



sighted " eyes, the eyeball is often too long from front to 

 back, so the rays meet in front of the retina. Concave 

 glasses remedy this defect. The eye may also be too 

 short (far-sighted) and need convex glasses. The refract- 

 ing surfaces (cornea and lens) may be unequally curved, 

 causing astigmatism. For most of these defects the 

 oculist can supply suitable glasses. 



